Some proposals have been made on a process for producing a film improved in gas barrier properties, hot water resistance and water vapor resistance by introducing an ionic bond between poly(meth)acrylic acid and a metal. The poly(meth)acrylic acid means polyacrylic acid, polymethacrylic acid or a mixture thereof.
There has been proposed, for example, a process for producing a gas barrier film improved in hot water resistance and water vapor resistance by subjecting a coating film formed of a mixture of poly(meth)acrylic acid and polyvinyl alcohol or a saccharide to a heat treatment to form a film and then subjecting the film to a dipping treatment in a medium containing an alkaline metal or alkaline earth metal to introduce an ionic bond between poly(meth)acrylic acid and the metal (U.S. Pat. No. 6,022,913; Document 1).
There has been proposed a process for producing a film excellent in gas barrier properties, hot water resistance and water vapor resistance by forming a metal compound-containing layer on the surface of a coating film formed from a mixture of poly(meth)acrylic acid or a partially neutralized product thereof and polyvinyl alcohol or a saccharide and forming an ionic bond by migration of the metal compound into the coating film (U.S. Pat. No. 6,605,344; Document 2).
According to the processes described in Documents 1 and 2, however, it is necessary to subject the coating film containing the mixture to a heat treatment over a relatively long period of time at a high temperature of at least 100° C. for obtaining a gas barrier film. In addition, the method for forming the ionic bond is complicated in these processes.
There has recently been proposed a process for forming a polyvalent metal salt of a polycarboxylic acid by a reaction of a carboxyl group of a polycarboxylic acid polymer and a polyvalent metal compound by forming a multi-layer film with a layer of the polycarboxylic acid polymer and a layer containing the polyvalent metal compound arranged adjacently to each other and put the multi-layer film under an atmosphere of a relative humidity of at least 20%, thereby causing a polyvalent metal ion to migrate into the polycarboxylic acid polymer layer from the polyvalent metal compound-containing layer (U.S. Patent Application Publication No. 2005/0131162; Document 3). According to this process, a film excellent in gas barrier properties can be obtained.
However, the process described in Document 3 requires a step of synthesizing the polycarboxylic acid polymer by polymerizing an α,β-unsaturated carboxylic acid monomer such as (meth)acrylic acid; a step of applying a coating liquid containing the polycarboxylic acid polymer and a step of applying a coating liquid containing the polyvalent metal compound, and so the operation is complicated. In addition, this process requires putting the multi-layer film over a long period of time under a high-humidity atmosphere for causing the polyvalent metal ion to migrate into the polycarboxylic acid polymer layer from the polyvalent metal compound-containing layer, and so it is difficult to continuously operate the process.
Document 3 also discloses a process for producing a gas barrier film by applying and drying an aqueous coating liquid containing a mixture of a polycarboxylic acid polymer and a polyvalent metal compound. However, the polycarboxylic acid polymer and the polyvalent metal compound are easy to react in an aqueous solution to cause insoluble precipitate, and so it is difficult to prepare an aqueous coating liquid with the respective component uniformly dissolved therein. When a volatile amine such as aqueous ammonia is added upon the preparation of the aqueous coating liquid, the reaction between the polycarboxylic acid polymer and the polyvalent metal compound can be inhibited. Since there is a need of volatilizing off the volatile amine after the application of the aqueous coating liquid, however, a working environment may possibly be adversely affected.